1) Field of the Invention
The subject invention generally relates to a lignocellulosic composite material and a method for preparing the lignocellulosic composite material. The subject invention also generally relates to a binder resin having at least one of an insecticide and a fungicide therein for forming the composite material.
2) Description of Related Art
Composite materials, such as oriented strand board (OSB), medium density fiberboard (MDF), agrifiber board, particle board, flakeboard, and laminated strand board (LVL) are known in the art. Generally, these types of boards are produced by blending or spraying lignocellulosic particles or materials with a binder resin while the lignocellulosic particles are tumbled or agitated in a blender or like apparatus. Lignocellulosic particles generally refer to wood particles as appreciated by those skilled in the art. After blending sufficiently to form a uniform mixture, the particles are formed into a loose mat, which is compressed between heated platens or plates, or by steam injection between the two platens to cure the binder and bond the flakes, strands, strips, pieces, etc., together in densified form. Conventional processes are generally carried out at temperatures of from about 120 to 225° C. in the presence of varying amounts of steam, either purposefully injected into or generated by liberation of entrained moisture from the wood or lignocellulosic particles. These processes also generally require that the moisture content of the lignocellulosic particles be between about 1 and about 20% by weight, before it is blended with the binder resin to produce adequate physical properties of the composite material.
The lignocellulosic particles can be in the form of chips, shavings, strands, wafers, fibers, sawdust, bagasse, straw, wood wool, bamboo and the like, depending upon the type of composite material desired to be formed. When the particles are larger, the boards produced by the process are known in the art under the general term of engineered wood. These engineered woods include panels, plywood, laminated strand lumber, OSB, parallel strand lumber, and laminated veneer lumber. When the lignocellulosic particles are smaller, the boards are known in the art as particleboard and fiber board.
The engineered wood products were developed due to the increasing scarcity of suitably sized tree trunks for cutting lumber. Such products can have advantageous physical properties such as strength and stability. Another advantage of the engineered wood and particle boards is that they can be made from the waste material generated by processing other wood and lignocellulosic materials. This leads to efficiencies and energy savings from recycling processes, and saves landfill space.
Binder resin compositions that have been used in making such composite wood products include phenol formaldehyde resins, urea formaldehyde resins, melamine urea formaldehyde, and isocyanates resins. Isocyanate binders are commercially desirable because they have low water absorption, high adhesive and cohesive strength, flexibility in formulation, versatility with respect to cure temperature and rate, excellent structural properties, the ability to bond with lignocellulosic materials having high water contents, and no additional formaldehyde emissions from resin. The disadvantages associated with the use of isocyanates include difficulty in processing due to their high reactivity, too much adhesion to platens, lack of cold tack, high cost and the need for special storage.
It is known to treat lignocellulosic materials with polymeric diphenylmethane diisocyanate (polymeric MDI or PMDI) to improve the strength of the composite material. Typically, such treatment involves applying the isocyanate to the material and allowing the isocyanate to cure, either by application of heat and pressure or at room temperature. While it is possible to allow the polymeric MDI to cure under ambient conditions, residual isocyanate groups remain on the treated products for weeks or even months in some instances. It is also known, but generally less acceptable from an environmental standpoint, to utilize toluene diisocyanate for such purposes. Isocyanate prepolymers are among the preferred isocyanate materials that have been used in binder compositions to solve various processing problems, particularly adhesion to press platens and high reactivity.
In the past, various solvents have been added to binder resin with the aim of achieving a lower viscosity and better handling properties. After application, the solvent evaporates during the molding process, leaving the bound particles behind. One major disadvantage of prior art solvents is that they cause a reduction in the physical properties of the formed board including a reduction in the internal bond strength of the formed board.
Separately from the formulation of improved lignocellulosic composite materials, it is desirable to prevent insects from damaging the composite materials over time and during normal use. Those skilled in the art of insecticides have developed numerous insecticides that are capable of killing or intoxicating various insects once they are exposed to the insecticide.
While these insecticides have been very commercially successful in the agricultural applications, typical applications have encountered difficulty in applying them in lignocellulosic composite materials. Various methods have been employed to incorporate these insecticides into the wooden structures discussed above and any other wooden article. For example, various prior art methods dissolve an insecticide in a solvent, such as water, and spray the solution onto the wooden structure. The solvent then absorbs into the wood and prevents the insects from damaging the wooden structure. However, one drawback with spraying the solution on wood that is already formed is that over time, the insects will eat away at the wood and eventually get beyond the point where the solution has absorbed. At this point, the wooden structure is vulnerable to subsequent attacks by insects. Another drawback to this method is that any additional water added during formation of the composite material reduces the physical properties of the final composite material. During the pressing stage, steam pressure from any water present in the composite material tends to reduce the physical properties. Therefore, adding additional water would increase the steam pressure and further reduce the physical properties. Additionally, it is typical to dry the wood strands to lower moisture content at the beginning to minimize this effect, but this additional drying costs energy and time.
Other methods, especially used in the formation of plywood, include incorporating a powder insecticide directly into a glue or an adhesive. Plywood, or laminated veneer, is prepared by applying glue to an already formed layer of wood and compressing it together with another layer of wood. The glue, having the insecticide therein, is applied between the layers of the wood and is compressed to form the plywood. However, the insecticide is not present, i.e., dispersed, throughout the wood, since it is only located in the glue between the layers. Therefore, it is possible to have an initial infestation of insects eat through the glue layer exposing the unprotected wood underneath. Subsequent infestations of insects are then able to cause substantial damage because the insecticide has been removed. In this method, the plywood has not been made insect resistant, only the glue is insect resistant.
Still other methods have incorporated the insecticide by encapsulating the insecticide in a polyurethane. It is known that the dispersibility and dissolvability of certain insecticides, such as fipronil, is difficult to achieve in certain substances, such as water. Therefore, encapsulating the insecticide in polyurethane improves the dispersibility of the insecticide. However, the encapsulation restricts the direct contact of the insecticide with the insect and requires the insect, in addition to eating the wood, to eat through the polyurethane prior to reaching the insecticide. Therefore, encapsulating the insecticide is not desirable. Further, the additional steps required to encapsulate the insecticide increase the time and cost of production, which are commercially unacceptable.
Fungicides have also been used to treat lignocellulosic composite materials. Fungicides are substances possessing the power of killing or preventing the growth of fungus. Therefore, the fungicides reduce the likelihood that the composite material will decay as a result of fungus over time. However, the application of the fungicide has been limited in similar circumstances as the insecticides discussed above.
Accordingly, it would be advantageous to provide a lignocellulosic composite material that is insect and fungus resistant and that is capable of withstanding insect attacks over a longer period of time to prevent insect damage to the composite material. The related art methods that only apply the insecticide to the surface of the wood or in the adhesive layers between the wood are subject to subsequent insect attacks after the insecticide layer has been breached. Therefore, it is desirable to produce a lignocellulosic composite material that has the insecticide present in a low dosage and dispersed throughout the composite material for preventing insect attacks.